A primary current focus in the study of depression is the means by which attachment disruption in childhood and other forms of early adversity increase vulnerability for developing major depressive disorder in later life. The mechanism for this effect remains obscure but is thought to involve sensitization of stress-related processes. A second recent emphasis in depression research has been the role of proinflammatory factors in mediating depressive symptoms. Research proposed here uses a guinea pig model to examine the possibility that proinflammatory activity induced by the stressor of maternal separation mediates a prolonged sensitization of depressive-like behavior. Guinea pigs exhibit a specific attachment process, display a depressive-like behavioral response to maternal separation that is mediated by proinflammatory activity, and show a sensitization of depressive-like behavior upon repeated separation that is accompanied by a sensitized core temperature response. Work proposed here will first determine if a prolonged (9 day) sensitization identified during the last funding period is mediated by a sensitization of proinflammatory activity (either enhanced proinflammatory response or increased sensitivity to a proinflammatory response (Aim 1). This will be done by attempting to block sensitization of depressive-like behavior and core temperature increase with anti-inflammatory agents (Interleukin-10 and indomethacin) and by determining whether prior separation increases the central cytokine and/or behavioral response to later immune challenge. Next, we will assess whether the sensitized behavioral response reflects sensitization of a broader underlying depressive-like state mediated by proinflammatory activity (Aim 2). This will be accomplished by determining if early separation enhances responsiveness in a validated behavioral test of depressive-like behavior (forced swim) that is independent of the initial inducing agent (separation), and whether the forced swim effect can be reversed by an anti-inflammatory compound.